In vivo three-dimensional mapping of biologic tissue and vasculature is a challenging proposition due to the highly-scattering and absorptive nature of biologic tissue. Some current methods have slow scanning speeds making in vivo three-dimensional imaging difficult. Some other techniques having faster scanning speeds are still lacking due to their inability to scan deeply into biologic tissue without producing overlapped images, requiring the use of invasive procedures to scan the tissue of interest. Many techniques aimed at deeper imaging generally cannot provide deep imaging of tissue having moving material (e.g., blood flow). Therefore, methods to effectively image structure and/or tissue movement, such as blood flow, are of substantial clinical importance.
Optical coherence tomography (OCT) is an imaging modality for high-resolution, depth-resolved cross-sectional, and 3-dimensional (3D) imaging of biological tissue. Among its many applications, ocular imaging in particular has found widespread clinical use. In the last decade, due to the development of light source and detection techniques, Fourier-domain OCT, including spectral (spectrometer-based) OCT and swept-source OCT, have demonstrated superior performance in terms of sensitivity and imaging speed over those of time-domain OCT systems. The high-speed of Fourier-domain OCT has made it easier to image not only structure, but also blood flow. This functional extension was first demonstrated by Doppler OCT which images blood flow by evaluating phase differences between adjacent A-line scans. Although Doppler OCT is able to image and measure blood flow in larger blood vessels, it has difficulty distinguishing the slow flow in small blood vessels from biological motion in extravascular tissue. In the imaging of retinal blood vessels, Doppler OCT faces the additional constraint that most vessels are nearly perpendicular to the OCT beam, and therefore the detectability of the Doppler shift signal depends critically on the beam incident angle. Thus, other techniques that do not depend on beam incidence angle are particularly attractive for retinal and choroidal angiography.
Several OCT-based techniques have been successfully developed to image microvascular networks in human eyes in vivo. One example is optical microangiography (OMAG), which can resolve the fine vasculature in both retinal and choroid layers. OMAG works by using a modified Hilbert transform to separate the scattering signals from static and moving scatters. By applying the OMAG algorithm along the slow scanning axis, high sensitivity imaging of capillary flow can be achieved. However, the high-sensitivity of OMAG requires precise removal of bulk-motion by resolving the Doppler phase shift. Thus, it is susceptible to artifacts from system or biological phase instability. Other related methods such as phase variance and Doppler variance have been developed to detect small phase variations from microvascular flow. These methods do not require non-perpendicular beam incidence and can detect both transverse and axial flow. They have also been successful in visualizing retinal and choroidal microvascular networks. However, these phase-based methods also require very precise removal of background Doppler phase shifts due to the axial movement of bulk tissue. Artifacts can also be introduced by phase noise in the OCT system and transverse tissue motion, and these also need to be removed.
To date, most of the aforementioned approaches have been based on spectral OCT, which provides high phase stability to evaluate phase shifts or differentiates the phase contrast resulting from blood flow. Compared with spectral OCT, swept-source OCT introduces another source of phase variation from the cycle-to-cycle tuning and timing variabilities. This makes phase-based angiography noisier. To use phase-based angiography methods on swept-source OCT, more complex approaches to reduce system phase noise are required. On the other hand, swept-source OCT offers several advantages over spectral OCT, such as longer imaging range, less depth-dependent signal roll-off, and less motion-induced signal loss due to fringe washout. Thus an angiography method that does not depend on phase stability may be the best choice to fully exploit the advantages of swept-source OCT. In this context, amplitude-based OCT signal analysis may be advantageous for ophthalmic microvascular imaging.
One difficulty associated with OCT's application in microvascular imaging comes from the prevalent existence of speckle in OCT images obtained from in vivo or in situ biological samples. Speckle is the result of the coherent summation of light waves with random path lengths and it is often considered as a noise source which degrades the quality of OCT images. Various methods have been developed to reduce speckle in spatial domain, such as angle compounding, spectral compounding, and strain compounding. Speckle adds to “salt-and-pepper-like” noise to OCT images and induces random modulation to interferometric spectra which can significantly reduce contrast.
In spite of being a noise source, speckle also carries information. Speckle pattern forms due to the coherent superposition of random phasors. As a result of speckle, the OCT signal becomes random in an area that is macroscopically uniform. If a sample under imaging is static, the speckle pattern is temporally stationary. However, when photons are backscattered by moving particles, such as red blood cells in flowing blood, the formed speckle pattern will change rapidly over time. Speckle decorrelation has long been used in ultrasound imaging and in laser speckle technique to detect optical scattering from moving particles such as red blood cells. This phenomenon is also clearly exhibited by real-time OCT reflectance images. The scattering pattern of blood flow varies rapidly over time. This is caused by the fact that the flow stream drives randomly distributed blood cells through the imaging volume (voxel), resulting in decorrelation of the received backscattered signals that are a function of scatterer displacement over time. The contrast between the decorrelation of blood flow and static tissue may be used to extract flow signals for angiography.
The speckle phenomenon has been used in speckle variance OCT for the visualization of microvasculature. Speckle patterns at areas with flowing blood have a large temporal variation, which can be quantified by inter-frame speckle variance. This technique termed “speckle variance” has been used with swept-source OCT demonstrating a significant improvement in capillary detection in tumors by calculation of the variance of the OCT signal intensity. A key advantage of the speckle variance method is that it does not suffer from phase noise artifacts and does not require complex phase correction methods. Correlation mapping is another amplitude-based method that has also recently demonstrated swept-source OCT mapping of animal cerebral and human cutaneous microcirculation in vivo. These amplitude-based angiography methods are well suited to swept-source OCT and offer valuable alternatives to the phase-based methods. However, such methods still suffer from bulk-motion noise in the axial dimension where OCT resolution is very high. Therefore, an amplitude-based swept-source angiography method that is able to reduce bulk-motion noise without significant sacrifice in the flow signal would be optimal. For example, imaging of retinal and choroidal flow could be particularly improved with such noise reduction, as in the ocular fundus the flow signal is predominantly in the transverse rather than axial dimension.